1. Field of Invention
The present invention relates to a driving device for driving an active matrix LCD (Liquid Crystal Display) panel.
2. Discussion of Background
An active matrix LCD device has a liquid crystal interposed between a common electrode and a plurality of pixel electrodes. Each pixel electrode is provided with an active device such as a TFT (Thin Film Transistor) and the active device is used to control whether a voltage of a source line is to be set for the pixel electrode.
The common electrode is set at a predetermined potential and each pixel electrode is set at a potential according to each pixel value of a display image. A state in which the potential of the pixel electrode is higher than the potential of the common electrode will be referred to as positive polarity. Furthermore, a state in which the potential of the pixel electrode is lower than the potential of the common electrode will be referred to as negative polarity.
FIG. 29 is an explanatory drawing showing an example of the potential of the common electrode and potentials to set the pixel in white or in black by each of polarities. The below will describe an example of the normally white case. The potential of the common electrode is denoted by VCOM. In FIG. 29, Vpb, Vpw, VCOM, Vnw, and Vnb represent respective potentials, which are in the relation of Vnb<Vnw<VCOM<Vpw<Vpb. For displaying the pixel in black by positive polarity, the potential of the source line connected to the pixel is set at Vpb; for displaying the pixel in white by positive polarity, the potential of the source line connected to the pixel is set at Vpw. For setting the pixel in halftone display by positive polarity, the potential of the source line connected to the pixel is set at a potential higher than Vpw and lower than Vpb. For displaying the pixel in black by negative polarity, the potential of the source line connected to the pixel is set at Vnb; for displaying the pixel in white by negative polarity, the potential of the source line connected to the pixel is set at Vnw. For setting the pixel in halftone display by negative polarity, the potential of the source line connected to the pixel is set at a potential lower than Vnw and higher than Vnb.
The active matrix LCD device is preferably driven so as to minimize consecutive arrangement of pixels of the same polarity, for prevention of crosstalk. FIG. 30 is an explanatory drawing showing a general LCD device. As shown in FIG. 30, pixel electrodes 50 are arranged in a matrix pattern and each pixel electrode is provided with a TFT 51. In FIG. 30, pixels for red display are denoted by “R,” pixels for green display by “G,” and pixels for blue display by “B.”
As shown in FIG. 30, the device is provided with a source driver 60 for setting potentials of the respective source lines S1 to Sn and the source lines are connected to respective output terminals D1 to Dn of the source driver 60. In the example shown in FIG. 30, each TFT 51 is disposed on the left side of the pixel electrode 50 and is connected to the source line present on the left side of the pixel electrode 50. Furthermore, gate lines G1, G2, G3, . . . are provided for respective rows of pixels and each gate line is connected to TFTs 51 of the respective pixel electrodes in the corresponding row. The gate lines are sequentially selected and the TFTs 51 in the selected row make the pixel electrodes 50 conductive to the respective source lines. As a consequence, the pixel electrodes 50 in the selected row are controlled to potentials equal to those of the source lines present on the left side of the pixel electrodes. The TFTs 51 in non-selected rows keep the pixel electrodes 50 nonconductive to the source lines. As the gate lines are sequentially selected, the source driver 60 sets the potentials of the respective source lines to potentials according to pixel values of the respective pixels in each selected row, thereby displaying an image according to image data.
In the general LCD device shown in FIG. 30, the source driver 60 controls the polarities of adjacent pixels so as to be different from each other, for example, as described below. During selection of a gate line of an odd-numbered row in a certain frame, the source driver 60 sets potentials of the source lines S1, S3, S6, . . . of the odd-numbered columns to potentials higher than the potential VCOM of the common electrode (not shown) and sets potentials of the source lines S2, S4, S6, . . . of the even-numbered columns to potentials lower than VCOM. During selection of a gate line of an even-numbered row, the source driver 60 sets potentials of the source lines S1, S3, S5, . . . of the odd-numbered columns to potentials lower than VCOM and sets potentials of the source lines S2, S4, S6, . . . of the even-numbered columns to potentials higher than VCOM. As a consequence, the display panel is controlled to make adjacent pixels alternately positive and negative, as shown in FIG. 30. In FIG. 30, “+” represents positive polarity and “−” negative polarity.
Furthermore, the source driver 60 switches the potentials of the source lines so as to invert the polarities of the individual pixels at every switch of frame. Namely, in the next frame to the foregoing frame, the source driver 60 sets the potentials of the source lines of the odd-numbered columns to potentials lower than VCOM and sets the potentials of the source lines of the even-numbered columns to potentials higher than VCOM during selection of a gate line of each odd-numbered row. During selection of a gate line of each even-numbered row, the source driver 60 sets the potentials of the source lines of the odd-numbered columns to potentials higher than VCOM and sets the potentials of the source lines of the even-numbered columns to potentials lower than VCOM. As a result, the polarities of the respective pixels become opposite to those of the pixels shown in FIG. 30.
In this driving method, every time the selected row is switched, the potentials of the individual source lines are varied from the potentials higher than VCOM to the potentials lower than VCOM or from the potentials lower than VCOM to the potentials higher than VCOM. For this reason, power consumption becomes greater. Particularly, since power consumption of the LCD panel is proportional to the square of a potential difference in each source line upon switching of the selected row, the increase in the number of potential switching times of the source lines leads to increase in power consumption.
There is a proposed LCD device capable of implementing control to make the polarities of adjacent pixels different, while reducing power consumption (cf. Patent Document 1). In the LCD device described in Patent Document 1, the TFTs connected to the gate lines of the odd-numbered rows are formed on the left side of the source lines and the TFTs connected to the gate lines of the even-numbered rows are formed on the right side of the source lines. This configuration prevents the potentials of the source lines from varying from potentials higher than VCOM to potentials lower than VCOM or from varying from potentials lower than VCOM to potentials higher than VCOM, at every select period.